A known power supply employs a thyristor, or a silicon controlled rectifier (SCR), in each phase of an alternating current main supply. At least two of the thyristors are always ON at any given time to conduct current to an inductor which stores energy and smooths the output. The other thyristors are sequentially turned ON, to control the average current flow, by means of a predictive control circuit, which attempts to predict the current demand over the following cycle. The thyristors are turned OFF by the next current zero to arrive.
This supply has a number of disadvantages. The first is that control is only exercised over the current at the times when the thyristors are being turned ON. This implies an average delay in the current control of a third of a period of the supply (when a thyristor is used in each phase of a three phase supply). It follows that there is a maximum rate at which current can be controlled. As a result, the inductance must be large enough to limit current ripple at higher rates. This is essential because current zeros extinguish the arc and high current peaks lead to electrode degradation. For example, a 50 kW arc torch consuming 200 Amps will need an inductor of 20 mH, which would weigh several tons, to limit current ripple to less than 50 Amps. This adds greatly to the expense of the power supply.
A second disadvantage arises from the fact that the switching control is predictive and results from a calculated guess rather than being absolutely determined from the current actually flowing at any given time.